1. Field of the Invention
The present invention relates to a step-up power supply device for supplying a DC voltage to various electronic devices and specifically relates to a step-up power supply device which includes a switchable step-up converter.
2. Description of the Prior Art
In recent years, the switchable step-up converter has been used as a step-up power supply device in various electronic devices which operate on batteries used as power supplies because of its highly-efficient power conversion characteristics. Since the voltage of the battery depends on the energy remaining in the battery, a lower input voltage of the step-up power supply device has been generally used for the purpose of using the electronic device for long hours without replacing or charging the battery. For example, in the case of two AA batteries connected in series, there is a demand that a low battery voltage, which is as low as the lower limit operation voltage of 1.5 to 1.8 V, is boosted for use in electronic devices.
An example of such a step-up power supply device is disclosed in Japanese Laid-Open Patent Publication No. 2003-92873. FIG. 7 shows a circuit structure of the step-up power supply device disclosed in FIG. 1 of this publication. In FIG. 7, the step-up power supply device includes an input power supply 1, a step-up converter 20, a startup circuit 30, and a control circuit 40. The step-up converter 20 includes an inductor 2, a switch element (bipolar transistor) 3, a diode 4, and an output capacitor 5. The startup circuit 30 which carries out a switching operation based on a low input voltage to supply drive signal V3 which is used for controlling the state (on/off) of the switch element 3 of the step-up converter 20 such that output voltage Vo of the step-up converter 20 is boosted. The control circuit 40 controls the on/off conduction ratio (duty ratio) based on input voltage Vi which is equal to or higher than reference voltage Vr.
The inputs of the startup circuit 30 and step-up converter 20 are connected to the input power supply 1. The input of the control circuit 40 is connected to the output of the step-up converter 20. Drive signals V3 and V4 respectively output from the startup circuit 30 and the control circuit 40 drive the current in the base of the switch element 3 of the step-up converter 20. A voltage supply circuit 50 generates reference voltage Vr. A comparator 51 compares output voltage Vo and reference voltage Vr. The output of the comparator 51 serves as a startup signal for the control circuit 40 and also serves as a startup signal for the startup circuit 30 via an inverter 52. When output voltage Vo is lower than reference voltage Vr, the comparator 51 outputs L-level to cause the startup circuit 30 to operate and, meanwhile, stop the control circuit 40 in normal driving mode so that its output terminal of drive signal V4 is rendered floating. When, to the contrary, output voltage Vo is higher than reference voltage Vr, the comparator 51 outputs H-level to cause the control circuit 40 to operate and, meanwhile, stop the startup circuit 30 so that its output terminal of drive signal V3 is rendered floating.
With the above structure, even if input voltage V1 from the input power supply 1 is low, the startup circuit 30 outputs drive signal V3 which is used for controlling the state (on/off) of the switch element 3 till output voltage Vo of the step-up converter 20 is boosted up to reference voltage Vr. Accordingly, the control circuit 40 is caused to operate so that output voltage Vo of the step-up converter 20 reaches a target value.
However, if the step-up power supply device having the above-described conventional structure operates in a low input voltage condition, output voltage Vo need to be boosted up to reference voltage Vr. Therefore, the drive signal output from the startup circuit 30 which is used for controlling the state (on/off) of the switch element 3 has a large duty ratio. In the case of a high input voltage, the switching current flowing from the inductor 2 to the switch element 3 is large. Thus, an inrush current flows at the time of startup, which deteriorates the performance of the battery of the input power supply 1.
The switch element 3 is formed by a bipolar transistor and the drive voltage is clamped on 0.7 V, such that in the case of a low input voltage no current flows via a parasitic diode which exists in the drive stage of the startup circuit 30. However, the bipolar transistor produces a large drive loss, and a higher switching frequency is difficult to achieve.